After a well has been drilled and casing has been placed in the well, the next step typically undertaken to complete the well is to perforate the well. The perforations are ideally formed opposite a formation from which oil and gas will hopefully be obtained. If the formation is perhaps 100 feet thick, locating the formation along the cased hole is not too difficult. Moreover, perforations placement into the formation is not quite so critical in view of the relative thickness of the formation. Thus, if there are perforations above or below the formation, it typically may not cause too much of a problem. Considering this example; if it is known that the formation is about 100 feet thick, a tubing conveyed perforating (TCP) assembly having perforating guns spanning about 100 feet is lowered into the well. When the TCP assembly is positioned in the well, misalignment relative to the formation is not a great problem by virture of the relative large thickness of the formation.
In the situation where the producing formation is only 10 feet thick, proper positioning of the TCP assembly is much more important. Assume that the formation of interest has a thickness of 10 feet; in that instance, registration of the TCP assembly may be crucial. Assume that the formation of interest is located about 12,000 feet deep in a well which has been drilled 10,000 feet. An error of 1% in measuring the depth from the surface to the formation is an error of 100 feet, 10 times larger than the formation thickness. Any measurement error of this magnitude could easily cause the perforations to completely miss the location of this thin formation. Thus, it is very difficult to align the TCP assembly solely from measurements obtained from the surface as, for example, by measuring the length of tubing in the well. The tubing is subject to elongation, and wireline supported tools are also subject to elongation.
In the past, one technique to overcome this problem has involved the use of a radioactive logging tool to obtain a correlation log to locate the formation of interest. Thus, the radioactive logging tool is used to make measurement through the wall of the casing and the surrounding cement holding the casing in place. This sequence of operation involves lowering a radioactive logging tool on a wireline or logging cable to a depth sufficient to move the logging tool past the formation of interest. The logging tool provides continuous data output to the surface such that the data is evaluated, thereby determining the location of the formation of interest. When it is found, the depth of the logging tool in the well is then determined. This is difficult if the logging tool is at a significant depth, but there are procedures available such that the elongation of the supporting cable connected to the logging tool can be evaluated and a precise location is then obtained. Knowing this depth, the TCP assembly is then positioned in the well opposite the formation of interest. As an example, the tubing string and the TCP assembly affixed to the bottom can be lowered almost to the bottom of the well, significantly past the estimated location of the formation of the interest. The logging tool is then used to locate the formation. The logging tool is removed and tubing is also removed to adjust the location of the TCP assembly. This procedure is continued until the TCP assembly is located opposite the target formation. Then, detonation can be initiated. The TCP assembly is detonated by dropping a weight in the tubing string, actuation of a pressure signal for pressure actuated detonating devices or dropping an electric line in the tubing string to connect with the TCP assembly for detonation by electrically triggered means.
This procedure just described primarily involves locating the formation with the logging tool, movement of the tubing string to relocate the TCP assembly opposite the formation while removing the logging tool. The latter two steps may be reversed in sequence. It also requires the detonation sequence to be initiated by means well known in the art. As mentioned above, three typical systems used including the dropped weight, pressure actuated detonation, or electric signal detonation using electric line. This sequence of TCP assembly positioning can require substantial amounts of rig time.
By contrast, the method of the present disclosure enables the formation to be located through the use of a logging tool, the logging tool being left in the tubing string even after the formation has been located. Moreover, a radioactive logging tool of conventional construction and supported on a conventional logging cable is provided with an electric line connector cooperative with a mating connector at the top end of the TCP firing head. The TCP firing head is affixed to the TCP assembly above the perforating guns. With this arrangement of apparatus, the TCP assembly positioning sequence then is simplified. The TCP assembly is lowered on the tubing string to a depth greater than the location of the formation. The radioactive logging tool is then used to precisely locate the formation of interest. Recall that the radioactive logging tool is able to find the formation through the casing and cement which isolates the well from the formation. As before, it is required to locate the formation also through the tubing as well as the casing. This can be readily accomplished. Once it is located and the depth of the formation is then noted, the tubing string is raised in the well to bring the TCP assembly into registration with the formation. This may require raising of the tubing string and is accompanied by raising of the logging tool also. However, they are only repositioned, not retrieved fully from the well. Once registration is obtained, the logging tool can then be lowered to make operative connection with a cooperative plug and socket whereby the electric line initiator is operatively connected to the TCP assembly. This enables a signal to be sent from the surface through an electric line to the TCP assembly for proper operation of the perforating guns.